1. Field of the Invention
The present invention relates to an automotive headlight of the projector type, and more particularly to a projector-type headlight of which the luminous intensity in a region near the boundary between a zone illuminated with the light frontwardly projected through a convex lens and a non-illuminated zone decreases gradually as going from the illuminated zone to the non-illuminated zone.
2. Description of the Related Art
Generally, the projector-type automomotive headlight of a car has such a luminous intensity pattern as to brightly illuminate the lane of travel of the car without dazzling the drivers of the cars running in the opposite direction. Such a luminous intensity pattern comes from the fact that the light projected from a lamp bulb and reflected by a reflector is shaped into an appropriate pattern by a shade located between the lamp bulb and a convex lens and then projected through the convex lens. The projector-type headlight of this kind has an optical system as schematically shown in FIGS. 1 and 2. As seen from these Figures, the reflector 10 has an inner surface formed as a reflecting surface constituting a part of a spheroid, the lamp bulb 12 as a light source is disposed at the inner focus of the spheroid and the shade 13 is located at the outer focus. This outer focus of the spheroid also lies in the focal plane of the convex lens 14 disposed in front of the reflector 10, The shade 13 has an edge formed along the meridional image surface i-j so that the reflected light from the reflecting surface of the reflector 10 is incident upon the meridional image surface i-j. The edge of the shade 13 has such a cut line as to let pass the upper half of the reflected light beam from the reflector 10 and a part of the lower half thereof while blocking the majority of the latter, whereby the light shaped by the shade 13 is projected frontwardly as converged by the convex lens 14. The lamp bulb 12 is fixed to the reflector 10, while the reflector 10, shade 13 and convex lens 14 are fixed on the frame 15. FIG. 3 is an isocandela diagram of a headlight of such a projector type. In this diagram, the horizontal angle of the light with respect to the axis of the reflector is taken horizontally while the vertical angle with respect to the reflector axis is taken vertically. For example, this isocandela diagram is presented by the curves resulting from connecting the points of equal luminous intensity on, for example, a screen placed outside the convex lens 13 and on which the light is projected from the headlight.
FIG. 4 shows isolux lines formed on the horizontal ground when the light is projected onto the ground from a headlight of such a projector type which is placed at a height of 65 cm from the ground level. In this diagram, the transverse distance from the light source is taken along the horizontal axis while the frontward distance from the light source is along the vertical axis. As seen from this diagram, the headlight of this projector type provides so definite a boundary between the illuminated and non-illuminated zones that when the driver turns his eyes from the illuminated zone to the non-illuminated zone, he will have a very dark field of vision in the latter zone. Such a lighting is apt to give the driver mental fatigue and anxiety. With the headlight of such a projector type, the driver cannot clearly see any walker crossing the lane of his car, for example, a pedestrian's crossing illuminated by a street light. Also, when the driver is going to turn along a curve with a small radius of turn, he will not clearly see the running course ahead of the curve.